This invention pertains, generally, to machines especially designed for the processing of pulp slurries, such as are used in paper-making, compaction baffle filters and twin roll presses being exemplary of such machines, and in particular to an inlet slice assembly for use with such machines.
Inlet slice assemblies are used to feed pulp slurry to compaction baffle filters and/or twin roll presses. They are employed to insure a uniform flow distribution over the total length of the subject machine. They have a metering opening for distributing slurry flow therethrough to the forming zone of the associated machine, the distribution being accomplished by the existence of a pressure differential of approximately five pounds per square inch between the inlet chamber of the inlet slice assembly and the forming zone of the associated machine. Means are provided for controlling the metering opening in response to the aforesaid pressure differential.
In addition, inlet slice assemblies deflocculate the stock, i.e., the pulp slurry. The slurry feed consistency is about two to eight percent. Pulp in this consistency forms flocs, i.e., small, spherical agglomerations of pulp. Flocs are undesirable for pulp washing because they will form an uneven mat with flow channels around the flocs, i.e., short circuiting of the wash water. In a twin roll press, the flocs are pressed into hard knits which are difficult to break up and dissolve. If undissolved, these hard knits will show up as undesirable "fish eyes" in the paper. The inlet slice assembly opening is widened periodically, e.g., every half hour or so, to clear any pulp or debris that might accumulate in the metering opening.
For normal operation, the metering opening of the slice assembly is approximately five to eight millimeters. At reduced tonnage and for smaller washer cylinder diameters, the opening is made even smaller. Floc size is approximately one and a half times the fiber length of the pulp. The average fiber length for softwood is about four millimeters. Thus, the floc size is about six millimeters in diameter.
The inlet slice assembly plugs almost immediately (within seconds) if the metering opening is smaller than the floc size. The aforesaid controlling means will compensate for the plugged slice assembly by making the metering opening larger. However, this defeats a principal purpose of the assembly, the effecting of a uniform flow across the full length of the machine, and will lead to plugging in the forming zone of the associated machine. The problem is worse for smaller cylinder diameters, such as in a twin roll press, because therein there is less flow per foot of roll length. It is also worse at reduced tonnage. Attempts to compensate for this have involved using a lower feed consistency at reduced tonnage to keep the flow high, but this results in more fines loss through the perforated drainage plates, and lower washing efficiency. The flow per foot is also lower for a slow-draining stock, but these stocks usually also have a shorter fiber length and smaller flocs.
The foregoing details the limitations known to exist in prior art. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the aforesaid limitations. Accordingly, a suitable alternative, embodied in a novel inlet slice assembly, is set forth herein, the same having features more fully disclosed hereinafter.